The present invention relates generally to the field of equipment for producing oil and gas, and more particularly to connections for joining conduits conducting the flow of oil and gas. Still more particularly, it relates to connections between subsea wellheads and subsea flowlines.
Finding and producing oil and gas continually becomes ever more complicated and expensive. New reserves of petroleum are harder to find and more difficult to produce with each passing year. The increasing scarcity of oil and gas long ago compelled the oil industry to move its search for hydrocarbons off dry land, to explore in offshore locations. The history of offshore exploration is marked by milestones of ever-increasing depth records for drilling and producing petroleum deposits. Inevitably, technology quite suitable for one set of conditions eventually becomes outmoded and inadequate when applied in deeper and harsher marine environments. The need for petroleum, however, continually pushes the limits of technology for providing safe, economical, controllable operations under more demanding conditions. Technology thus evolves to cope with these new demands.
One area witnessing major strides and steady advancement in capability is the technology of sea floor-based petroleum production. Wellheads can be placed upon the sea floor to control and direct the flow of oil and gas to remote processing facilities. Christmas trees, manifolds, and associated piping can be located at the subsea wellhead for direct regulation of the production process. Subsea Christmas trees often allow for remote control and manipulation of valves, chokes, regulators, etc., from a distant facility. Such isolated subsea wellheads thus allow for producing oil and gas reserves that otherwise would be uneconomical to recover because of environmental conditions, location and dispersion of reserves, and quantity of reserves recoverable. Subsea wellhead and production equipment, therefore, ultimately provide means for producing more of the earth's valuable petroleum deposits.
While known apparatus and methods for subsea production have solved numerous prior problems, significant operational obstacles still exist. In relatively shallow, calm waters, diving personnel can effectively and economically perform a multitude of underwater tasks necessary to initiate and maintain subsea production. In deeper-water locations, however, physiological limitations impose numerous restraints on the feasible use of diver assistance. Extended diving in deeper waters involves more complex equipment, such as diving bells, diving suits, or saturation chambers, to enhance the abilities and lengthen the allowable time of divers working at extreme depths. Ultimately, it becomes more cost-effective to design for installing and operating subsea equipment without need for diver assistance. It has become commonplace to connect subsea trees and flowlines remotely, without direct "hands-on" human intervention. New problems arise, however, using such diverless equipment. One common problem is the difficulty in achieving precise alignment and accurate construction of separate and discrete elements deep within the ocean. Conditions several thousand feet or more below the surface differ drastically from those within regulated, controlled onshore fabrication and assembly locations. Slight misalignments of independently-machined parts can be overly costly to a subsea project, since they can result in the need for complex and time-consuming emergency repairs or other remedial action, both above and below the surface of the sea. One solution to the misalignment problem is to maintain manufacturing tolerances to extremely narrow limits, and to account in the design for all conditions expected to be experienced in the final stages of subsea construction. Such tight tolerances and all-encompassing, complex design, however, are inordinately expensive and difficult to carry out in practice. Expensive design and fabrication of equipment can drive up costs enough to defeat the economic justification for a subsea petroleum production project.
What were relatively simple tasks to accomplish in less-demanding locations, moreover, can become difficult or nearly impossible in more severe environments. Onshore, for example, joining two fluid-carrying pipes together and sealing the contained internal pressure is easily accomplished with a conventional API sealed flange having studs, nuts, and a ring gasket. The ring gasket seals the connection; the studs and nuts exert the forces needed to hold the connection together. Aligning flanges, inserting a gasket, and threading and tightening studs and nuts can, however, become an exorbitant and impractical task at great subsea depths. The difficulty in connecting flow conduits worsens as water depths and internal fluid pressures increase. API flanges and other conventional connections necessarily require forces to hold them together. The internal pressure tends to push two joined conduits apart at the point of connection. The internal pressure acting on the seal area between the joined conduits creates this so-called "blow-apart" force. Higher internal pressures create greater blow-apart forces. Higher internal pressures thus require larger connections with larger, additional hardware to overcome the greater blow-apart forces. Mating and securing such larger connections without diver assistance therefore becomes increasingly complex at greater subsea depths.
Other problems that are minor or inconsequential in simpler environments become of major concern at great subsea pressures and depths. Wellheads are attached to the tops of vertical strings of well casing which can be hundreds or thousands of feet long extending deep into the earth. Flowlines and production equipment, by contrast, are secured at or near the sea floor, or even float in the water. Differential settling inevitably occurs, causing relative movement between a wellhead and the flowlines attached to it. Onshore, this problem is relatively minor, since the wellhead and Christmas tree are readily observable, and in most cases corrections or repairs can easily be made. For subsea production, however, the problem is much more serious. Differential settling can impose considerable stresses in the flowlines and the Christmas tree when they are joined by a rigid connection, such as a simple flange having very little, if any, "play" in it. As a result, when the flowlines or subsea tree are subjected to even minimal vertical or horizontal movements which occur due to settling, subsea currents, or routine operations, the probability of material or equipment failure increases. Again, the results of such failure are exacerbated and intensified in the subsea environment. Repairs are often difficult and time-consuming. Pollution and degradation of the marine environment also can result from subsea "spills" caused by failures in flow conduit piping or connectors, with the possibility of attendant legal liability of the operator or equipment manufacturer.
Subsea production technology is a vital link in the chain of supply of oil and gas so crucial to the functioning of modern industrial society. Prior art apparatus has provided means for subsea production, but significant problems remain. Existing equipment still confronts the user with the inherent disadvantages of having to achieve safe, controllable, precise subsea connections with relatively uneconomical or impractical prior art methods and apparatus. The need therefore exists for subsea flow conduit connections which are simple, yet reliable. Such connections need also to allow for relative movement of attached components, thereby reducing the operating hazards and the risks of failure of subsea flow connections.